1. Field of the Invention
The field of this invention resides within the field of ultrasonic wire bonding. Ultrasonic wire bonding is known for purposes of attaching wire interconnections to semiconductor devices either by ball bonding or wedge bonding. Such ball and wedge bonding is specifically performed by ultrasonic wire bonders that are attached to a support medium that moves them into a respective location for a proper bond. This particular invention is more finely delineated to be within the wedge bonding field.
2. The Prior Art
Wedge bonding for semiconductor devices is known in the art. Such bonding employs machinery and methods with a wire oftentimes formed of aluminum. The aluminum wire is connected from one point to another. The diameter of such wire in many cases can range from between 0.001 to 0.025 inches.
The wire is pressed against the surface of a semiconductor chip with a bonding tool. The end of the tool is vibrated with ultrasonic vibrations in a plane of motion generally parallel to the surface of the semiconductor to which a bond is to be formed. This ultrasonic vibration is for a period of tens of milliseconds. The combination of a static load on the bonding tool normal to the chip's surface to which the wire is bonded and the vibration of the end of the tool parallel to the surface cause the wire to plastically deform. As the wire plastically deforms, it simultaneously joins with the atoms of the material composing the chip's surface to provide a cold weld.
The invention hereof has two major elements. Firstly, it comprises a method of controlling the position and formation of the bonding wire and secondly a method for cutting the wire after the last bond. Other methods are currently used. This invention obviates those methods by eliminating the need for additional active devices. Instead it uses the basic mechanism of the equipment itself with the novel method hereof to perform both tasks of holding the wire and cutting it.
A common feature of many wedge bonding machines and tools is a clamp to hold the wire in a proper position under the tool tip. This is necessary while the tool is moving from one point where one wire has been terminated to a point where a second wire will be bonded. This prevents the wire from being removed from the tip while maintaining it for the second bond. Such a clamp is generally an active device employing a solenoid connected to a pair of jaws forced to open and close on command with respect to the particular bonding position or placement of the wire. Such a holding of wires is generally effective. However, the solenoid adds mass to the bonding head assembly that must be low in mass for fast operation and to minimize wire flattening from decelleration forces as the wire is pressed against the work surface.
The concept of such an active clamp also requires drive circuitry and means to supply the energy to the solenoid. The clamp is a necessary but troublesome feature of a bond head. It has been thought that if a method for eliminating it could be employed, wire bonding could be substantially enhanced.
Another drawback is that the clamp oftentimes occupies space behind or near the wire bonding tool. This restricts the use of the bonder to relatively open space areas clear of other components. This is due to the tool and clamp requiring space in which they can penetrate when moved to a component.
In order to avoid the occupying of space behind the tool, some equipment designs feature clamping means remotely positioned from the tool. This is somewhat disadvantageous inasmuch as the further the clamp is positioned from the tool, the more difficult it is to maintain a proper tail length to the wire after it has been bonded. The bond tail is that portion of unbonded wire projecting ahead of the first bond on a wire. This tail should be as short as practical, yet not less than the desired length of the bond.
The invention hereof eliminates the need for a separate clamp power source. The invention occupies a small space above and behind the bonding tool. This permits access into restricted space occupied by other elements of the component.
Additionally, the invention removes spool set or curvature to the wire caused by the wire having been coiled onto relatively small diameter spools. This is most important for accurate wire handling and positioning.
The invention further improves loop formation in the wire between bonds. This is desirable to avoid stresses in the wire lead and ultimately to device failure. If the loop is properly formed, it allows for expansion and contraction movements. Thus, proper loop formation is desirable and is enhanced significantly by this invention.
Repeatable looping requires bending the wire slightly but continuously as the tool rises from the first bond point. This minimally sets or bends the wire so that it will bend still further in the same direction as the machine moves between the first bond point and second bond point. It is important that the condition of the wire with respect to prior coil set be uniform. In measurable effect, uniform loops will occur if the axial orientation of the coil set is contant. Current bonders sometimes have the wire twist between spool and tool, thus affecting the initial condition of the wire and hence the loop shape.
The same mechanism that normalizes the wire with respect to coil set also provides a very low and controlled drag on the wire as the wire pays out between bonds. This drag is sufficient to serve the same purpose as the wire clamp used in the prior art. Those designs clamped the wire between jaws when necessary to hold it during moves between successive wires. A drag force in excess of any force that would cause the wire to slip or be drawn by gravitational forces with respect to the tool is sufficient. The invention incorporates a simple passive drag system in place of a more complicated active wire clamp.
After initial bonding, withdrawal of wire from the spool, and the provision of the loop formation, the wire is then placed in a position for the bonding operation at a second location. At this second location, after bonding, the wire is then cut by means of a cutter. The cutter is a significant improvement over all prior bonding cutters as shall be shown hereinafter. It has a knife edge that drives against the surface of the wire to be cut without having attendant solenoid drive means and other devices for cutting the wire. It furthermore moves in relative relationship to the bonding tool so as to allow to cut without displacing the tool.
After the wire is cut, the wire can then be severed while simultaneously implacing the wire in a slot or groove at the end of the tool. This serves to cleat the wire in the tool so that it can then be placed for a subsequent bonding operation.
A further feature allows the cutter to move against a flexible underlying substrate to a relative position with said bonding head to provide a sufficient depth of cut. Afterwards the wire can then be severed.
All the foregoing features and advantages over the prior art shall be seen hereinafter in the description of the apparatus and method in conjunction with the drawings as set forth.